Conventionally, oxygen detecting devices have been used in a variety of fields of, e.g., food; printing ink; electrics, mechanics, and appliances; and medicaments and medical products, for determining quality preservation and for controlling quality of various products contained in a packaging container, the products including food; inks; electrics, mechanics, and appliances; medicines; and medical products.
For example, in the field of food, the oxygen detecting device mainly targets at foods to be preserved for a long period of time. In order to prevent deterioration due to oxidation of oils and vitamins contained in food, damages caused by insects, changes in color and gloss, and growth of mold or bacteria, the oxygen detecting device is enclosed chiefly with a deoxidizer (oxygen absorbent) in a gas-impermeable hermetically-sealed package bag containing such foods. In the package bag, the state is kept where there is substantially no oxygen for a long period of time. Irrespectively of the type of food, upon detection of a predetermined amount of oxygen entering the package bag from outside, the color of the oxygen detecting device changes uniformly.
That is, conventionally, a food, a deoxidizer, and an oxygen detecting device are enclosed together in the package bag, and the deoxidizer absorbs or reacts with oxygen entering the package bag from outside through the package bag, an enclosing seal portion, a pin hole on the package bag, etc. with a lapse of a long period of time during food preservation, to thereby reduce or remove oxygen in the package bag. The amount of oxygen remaining in the package bag (that is not absorbed to the deoxidizer or reacted with the deoxidizer) due to a reduction in performance of the deoxidizer or other factors is checked based on the presence or absence of a color change of the oxygen detecting device, which is used as an index for visually determining the degree of freshness of the packaged food (see, e.g., Patent Documents 1 to 6).    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2002-308342 A    Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2000-342235 A    Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2000-039429 A    Patent Document 4: Japanese Patent Application Laid-Open Publication No. H11-276888 A    Patent Document 5: Japanese Patent Application Laid-Open Publication No. H11-190729 A    Patent Document 6: Japanese Utility Model Application Laid-Open Publication No. H5-85623 A